edit: All right, all right! It's a fair cop.
The person wearing them could be on a call, and the caller on the other side would hear what you say.
The person wearing them could be streaming to twitch, and everybody watching that stream would hear what you say.
All while you assume that you are having a private conversation with earbud person.
Which I don't do. If someone is wearing earbuds, that's an unambiguous signal that they don't want people talking to them.
edit: All right, all right! It's a fair cop.